Tumor antigens are ideally positioned as biomarkers and drug targets, and they play a critical role in the development of novel strategies for active and passive immunotherapy agents, to be used as stand-alone therapies or in conjunction with conventional therapies for cancer. Tumor antigens can be classified as either tumor-specific antigens (TSAs) where the antigens are expressed only in tumor cells and not in normal tissues, or tumor-associated antigens (TAAs) where the antigens are overexpressed in tumor cells but nonetheless also present at low levels in normal tissues.
TAAs and TSAs are validated as targets for passive (antibody) therapy as well as active immunotherapy using strategies to break immune tolerance and stimulate the immune system. The antigenic epitopes that are targeted by these therapeutic approaches are present at the cell surface, overexpressed in tumor cells compared to non-tumor cells, and are targeted by antibodies that block functional activity, inhibit cell proliferation, or induce cell death.
There are growing number of tumor-associated antigens against which monoclonal antibodies have been tested or are in use as treatment for cancer. The identification and molecular characterization of novel tumor antigens expressed by human malignancies is an active field in tumor immunology. Several approaches have been used to identify tumor-associated antigens as target candidates for immunotherapy, including high throughput bioinformatic approaches, based on genomics and proteomics. The identification of novel TAAs or TSAs expands the spectrum of tumor antigen targets available for immune recognition and provides new target molecules for the development of therapeutic agents for passive immunotherapy, including monoclonal antibodies, whether unmodified or armed. Such novel antigens may also point the way to more effective therapeutic vaccines for active or adoptive immunotherapy.
Cancer vaccination involves the administration of tumor antigens and is used to break immune tolerance and induce an active T-cell response to the tumor. Vaccine therapy includes the use of naked DNA, peptides, recombinant protein, and whole cell therapy, where the patient's own tumor cells are used as the source of the vaccine. With the identification of specific tumor antigens, vaccinations are more often carried out by dendritic cell therapy, whereby dendritic cells are loaded with the relevant protein or peptide, or transfected with vector DNA or RNA.
The major applications of anti-TAA antibodies for treatment of cancer are therapy with naked antibody, therapy with a drug-conjugated antibody, and fusion therapy with cellular immunity. Ever since their discovery, antibodies were envisioned as “magic bullets” that would deliver toxic agents, such as drugs, toxins, enzymes and radioisotopes, specifically to the diseased site and leaving the non-target normal tissues unaffected. Indeed, antibodies, and in particular antibody fragments, can function as carriers of cytotoxic substances such as radioisotopes, drugs and toxins. Immunotherapy with such immunoconjugates is more effective than with the naked antibody.
In contrast to the overwhelming success of naked (such as Rituxan and Campath) and conjugated antibodies (such as Bexxar and Zevalin) in treating hematological malignancies, only modest success has been achieved in the immunotherapy of solid tumors. One of the major limitations in successful application of immunotherapy to solid tumors is the large molecular size of the intact immunoglobulin that results in prolonged serum half-life but in poor tumor penetration and uptake. Indeed, only a very small amount of administered antibody (as low as 0.01%) reaches the tumor. In addition to their size, antibodies encounter other impediments before reaching their target antigens expressed on the cell surface of solid tumors. Some of the barriers include poor blood flow in large tumors, permeability of vascular endothelium, elevated interstitial fluid pressure of tumor stroma, and heterogenous antigen expression.
With the advent of antibody engineering, small molecular weight antibody fragments exhibiting improved tumor penetration have been generated. Such antibody fragments are often conjugated to specific cytotoxic molecules and are designed to selectively deliver them to cancer cells. Still, solid tumors remain a formidable challenge for therapy, even with immunoconjugated antibody fragments.
The new wave of optimization strategies involves the use of biological modifiers to modulate the impediments posed by solid tumors. Thus, in combination to antibodies or their conjugated antibody fragments, various agents are being used to improve the tumor blood flow, enhance vascular permeability, lower tumor interstitial fluid pressure by modulating stromal cells and extracellular matrix components, upregulate expression of target antigens and improve penetration and retention of the therapeutic agent.
Immunotherapy with antibodies represents an exciting opportunity for combining with standard modalities, such as chemotherapy, as well as combinations with diverse biological agents to obtain a synergistic activity. Indeed, unconjugated mAbs are more effective when used in combination with other therapeutic agents, including other antibodies.
Another component of the immune system response to immunotherapy is the cellular response, specifically—the T cell response and activation of cytotoxic T cells (CTLs). The efficiency of the immune system in mediating tumor regression depends on the induction of antigen-specific T-cell responses through physiologic immune surveillance, priming by vaccination, or following adoptive transfer of T-cells. Although a variety of tumor-associated antigens have been identified and many immunotherapeutic strategies have been tested, objective clinical responses are rare. The reasons for this include the inability of current immunotherapy approaches to generate efficient T-cell responses, the presence of regulatory cells that inhibit T-cell responses, and other escape mechanisms that tumors develop, such as inactivation of cytolytic T-cells through expression of negative costimulatory molecules. Effective immunotherapy for cancer will require the use of appropriate tumor-specific antigens; the optimization of the interaction between the antigenic peptide, the APC and the T cell; and the simultaneous blockade of negative regulatory mechanisms that impede immunotherapeutic effects.
T-cell activation plays a central role in driving both protective and pathogenic immune responses, and it requires the completion of a carefully orchestrated series of specific steps that can be preempted or disrupted by any number of critical events. Naïve T cells must receive two independent signals from antigen-presenting cells (APC) in order to become productively activated. The first, Signal 1, is antigen-specific and occurs when T cell antigen receptors encounter the appropriate antigen-MHC complex on the APC. A second, antigen-independent signal (Signal 2) is delivered through a T cell costimulatory molecule that engages its APC-expressed ligand. In the absence of a costimulatory signal, T-cell activation is impaired or aborted, which may lead to a state of antigen-specific unresponsiveness (known as T-cell anergy), or may result in T-cell apoptotic death.
Costimulatory signals can be either stimulatory (positive costimulation) or inhibitory (negative costimulation or coinhibition). Positive costimulation is required for optimal activation of naïve T cells, while negative costimulation is required for the acquisition of immunologic tolerance to self, as well as the termination of effector T cell functions. Costimulatory signals, particularly positive costimulatory signals, also play a role in the modulation of B cell activity. For example, B cell activation and the survival of germinal center B cells require T cell-derived signals in addition to stimulation by antigen.
Both positive and negative costimulatory signals play critical roles in the regulation of cell-mediated immune responses, and molecules that mediate these signals have proven to be effective targets for immunomodulation. Based on this knowledge, several therapeutic approaches that involve targeting of costimulatory molecules have been developed, and were shown to be useful for prevention and treatment of cancer and autoimmune diseases, as well as rejection of allogenic transplantation, each by turning on, or preventing the turning off, of immune responses in subjects with these pathological conditions.
Costimulatory molecule pairs usually consist of ligands expressed on APCs and their cognate receptors expressed on T cells. The well characterized B7/CD28 and CD40/CD40L costimulatory molecules are critical in primary T-cell activation. In recent years, several additional costimulatory molecules have been identified, that belong to the B7/CD28 or the TNF/TNF-R gene families. The effects of costimulatory TNFR family members can often be functionally, temporally, or spatially segregated from those of CD28 family members and from each other. The sequential and transient regulation of T cell activation/survival signals by different costimulators may function to allow longevity of the response while maintaining tight control of T cell survival.
The B7 family consists of structurally related, cell-surface protein ligands, which bind to receptors on lymphocytes that regulate immune responses. Interaction of B7-family members with their respective costimulatory receptor, usually a member of the CD28-related family, augments immune responses, while interaction with coinhibitory receptors, such as CTLA4, attenuates immune responses. Members of the B7 family share 20-40% amino-acid identity and are structurally related, with the extracellular domain containing tandem domains related to variable and constant immunoglobulin domains.
There are currently seven known members of the family: B7.1 (CD80), B7.2 (CD86), B7-H1 (PD-L1), B7-H2 (ICOS-L), B7-DC (PD-L2), B7-H3, and B7-H4, each with unique, yet often overlapping functions. Clearly, each B7 molecule has developed its own indispensable niche in the immune system. As specific niches of B7 family members continue to be dissected, their diagnostic and therapeutic potential becomes ever more apparent. Many of the B7 superfamily members were initially characterized as T cell costimulatory molecules. However, more recently it has become clear they can also coinhibit T cell responses. Thus, B7 family members may have opposing effects on an immune response.
Central to the normal function of the immune system is its ability to distinguish between self and non-self, since failure to do so could provoke the onset of autoimmune disease. Most autoimmune disorders are known to involve autoreactive T cells and/or autoantibodies. Thus, agents that are capable of inhibiting or eliminating autoreactive lymphocytes have a promising therapeutic potential. Furthermore, the use of agents that exhibit such immunosuppressive activity should also be beneficial in order to inhibit normal immune responses to alloantigens in patients receiving a transplant. Thus, novel agents that are capable of modulating costimulatory signals, without compromising the immune system's ability to defend against pathogens, are highly advantageous for treatment and prevention of such pathological conditions.
The importance of the B7 family members in regulating immune responses to self and allo-antigens was demonstrated by the development of immunodeficiency and autoimmune diseases in mice with mutations in B7-family genes. Accordingly, manipulation of the signals delivered by B7 ligands has shown potential in the treatment of autoimmunity, inflammatory diseases, and transplant rejection. This approach relies, at least partially, on the eventual deletion of auto- or allo-reactive T cells, presumably because in the absence of costimulation (which induces cell survival genes) T cells become highly susceptible to induction of apoptosis.
Harnessing the immune system to treat chronic diseases is a major goal of immunotherapy. Active and passive immunotherapies are proving themselves as effective therapeutic strategies. Passive immunotherapy, using monoclonal antibodies or receptor Fc-fusion proteins, has come of age and has shown great clinical success. A growing number of such therapeutic agents have been approved or are in clinical trials to prevent allograft rejection or to treat autoimmune diseases and cancer. Active immunotherapy (i.e. vaccines) has been effective against agents that normally cause acute self-limiting infectious diseases followed by immunity and has been at the forefront of efforts to prevent the infectious diseases that plague humankind. However, active immunotherapy has been much less effective against cancer or chronic infectious diseases primarily because these have developed strategies to escape normal immune responses. Among these are negative costimulators of the B7 family, such as B7-H1 and B7-H4, which are highly expressed in certain tumors, and afford local protection from immune cells-mediated attack.
The efficiency of the immune system in mediating tumor regression depends on the induction of antigen-specific T-cell responses through physiologic immune surveillance, priming by vaccination, or following adoptive transfer of T-cells. Although a variety of tumor-associated antigens have been identified and many immunotherapeutic strategies have been tested, objective clinical responses are rare. The reasons for this include the inability of current immunotherapy approaches to generate efficient T-cell responses, the presence of regulatory cells that inhibit T-cell responses, and other escape mechanisms that tumors develop, such as inactivation of cytolytic T-cells through expression of negative costimulatory molecules. Effective immunotherapy for cancer will require the use of appropriate tumor-specific antigens; the optimization of the interaction between the antigenic peptide, the APC and the T cell; and the simultaneous blockade of negative regulatory mechanisms that impede immunotherapeutic effects.
Costimulators of the B7 family play a critical role in activation and inhibition of antitumor immune responses. Novel agents targeting these molecules could find significant use in the modulation of immune responses and the improvement of cancer immunotherapy. Such agents could be administered in conjunction with tumor-specific antigens, as an adjuvant that serves to enhance the immune response to the antigen in the patient. In addition, such agents could be of use in other types of cancer immunotherapy, such as adoptive immunotherapy, in which tumor-specific T cell populations are expanded and directed to attack and kill tumor cells. Agents capable of augmenting such anti-tumor response have great therapeutic potential and may be of value in the attempt to overcome the obstacles to tumor immunotherapy.
Passive tumor immunotherapy uses the exquisite specificity and lytic capability of the immune system to target tumor specific antigens and treat malignant disease with a minimum of damage to normal tissue. Several approaches have been used to identify tumor-associated antigens as target candidates for immunotherapy. The identification of novel tumor specific antigens expands the spectrum of tumor antigen targets available for immune recognition and provides new target molecules for the development of therapeutic agents for passive immunotherapy, including monoclonal antibodies, whether unmodified or armed. Such novel antigens may also point the way to more effective therapeutic vaccines for active or adoptive immunotherapy.
Clinical development of costimulation blockade came to fruition with the approval of CTLA4Ig (abatacept) for rheumatoid arthritis. This soluble fusion protein, which acts as competitive inhibitor of the B7/CD28 costimulatory pathway, is also in clinical trials for other immune diseases such as psoriasis and multiple sclerosis, and for transplant rejection. Promising results have also been obtained in a phase II clinical trial in kidney transplantation with belatacept, a re-engineered CTLA4Ig with enhanced binding affinity to its ligands, B7.1 and B7.2 (CD80 and CD86, respectively). Two fully human anti-CTLA4 monoclonal antibodies, Ipilimumab and tremelimumab, abrogate the CTLA4/B7 inhibitory interaction, and are in clinical phase III for metastatic melanoma and other cancers, as well as HIV infection. Galiximab is a primatized monoclonal antibody targeting CD80, in Phase II for rheumatoid arthritis, psoriasis and Non-Hodgkin's lymphoma.
It is important to point out that strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies may involve the simultaneous blockade of several selected pathways or combination therapy with conventional drugs, such as immunosuppressants for immune-related disorders or cytotoxic drugs for cancer.
Despite recent progress in the understanding of cancer biology and cancer treatment, as well as better understanding of the molecules involved in immune responses, the success rate for cancer therapy and for the treatment of autoimmune diseases remains low. Therefore, there is an unmet need for new therapies which can successfully treat both cancer and autoimmune disorders.